The invention relates to the fabrication of semiconductor devices, and more particularly, to a photolithography method for manufacturing a device by using part of the formed device as a mask and exposing a photoresist with light that passes through the other portion of the device.
The invention can be more easily understood with reference to the fabrication of insulating layers; however, it will be obvious to those skilled in the art from the following discussion that the invention may be utilized in a wide variety of processes. Semiconductor fabrication often requires the formation of an insulating layer structure aligned with the surface of semiconductor devices having microstructures on the order of xcexcm. For example, Yoshinori Kimura, et al. have disclosed a method for forming a semiconductor laser device using a GaN-based material (Jpn. J. Appl. Phys., Vol. 37 (1998), pp. L1231-L1233) in which a narrow optical waveguide with a width of 5 xcexcm or less must be fabricated. A photoresist that has been patterned to a width of about 5 xcexcm, or an etching mask material that has been patterned using this photoresist, is used as an etching mask. The underlying GaN-based material having the base pattern is etched to form a multi-mesa structure. With a device such as this, the area around the optical waveguide must be covered with an insulating material to insulate the portion around the optical waveguide from the electrode pad provided on top of this insulating material.
A number of prior art methods have been used to form such a structure. For example, a photoresist pattern can be photolithographically formed over the optical waveguide, the pattern being adjusted such that it is smaller than the optical waveguide. SiO2 or the like is then vapor-deposited over the entire surface, and then an organic solvent, a photoresist exfoliant, or the like is used to remove the SiO2 on the photoresist along with the photoresist in the regions of the underlying pattern that must exposed to make electrical contacts.
Alternatively, when the base pattern is formed, a second layer composed of another material is simultaneous patterned. These layers are then used as etching mask materials and the underlying GaN-based material is etched. An insulating film such as SiO2 is then vapor-deposited, and the second layer and overlying insulating film are removed together to form an insulating film pattern. For example, in GaN systems, the GaN-based material is etched by reactive ion etching with the photoresist used during the formation of the base pattern still intact. Then lift-off patterning of the SiO2 or other insulating film is performed using this photoresist as a lift-off material.
In the above-described methods, the photomask and the pattern formed first on the sample have to be properly aligned by a process that substantially increases the cost of fabrication. The alignment is typically checked manually using alignment marks on the sample and alignment marks on the photomask. An XY and rotary stage is then used to align the relative positions of the photomask and the sample until any misalignment is within tolerance. However, because an alignment margin of about 1 xcexcm is required, these methods can only be applied to a photomask that is about 1 to 2 xcexcm smaller than the base pattern.
The alignment may also be performed using an electron beam lithography system or a photo-stepper having a high-precision, automatic positioning system. However, the high cost of this equipment significantly increases the manufacturing cost.
To avoid the alignment costs, a SiO2 or other insulating film is vapor-deposited over the entire surface of a sample, and the insulating film is removed from just the desired area by photolithography and etching technology. For example, in the etching of the GaN-based material, the photoresist serving as the mask material on top thereof is also etched. Unfortunately, the resultant pattern is rounded off, particularly at the edges. In addition, the reaction products produced during the etching make it more difficult for the photoresist to be peeled away. The shape changes in the photoresist and the formation of reaction products lead to diminished yields, which, in turn, increase the cost of manufacture. While this method does not require an accurate mask-alignment step, it does require the selective removal of the SiO2 or other insulating film formed on the sample surface along with the photoresist after the etching of the GaN-based material.
Broadly, it is the object of the invention to provide an improved fabrication method for providing a patterned insulating layer over an underlying semiconductor device or the like.
It is a further object of the invention to provide a fabrication method that does not require a high-precision alignment operation.
It is a still further object of the invention to provide a fabrication method that does not require costly alignment equipment.
These and other objects of the invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The invention is a method for generating a photoresist pattern on top of an object that includes a layer of material that is opaque to light of a predetermined wavelength. The object is first covered with a layer of photoresist material. The layer of photoresist material is then irradiated with light of the predetermined wavelength from a position under the object such that the object casts a shadow into the layer of photoresist. The photoresist material is then developed to generate the photoresist pattern. The layer of photoresist material is irradiated from below the object by providing a reflecting surface below the object and a light source above the object. A mask is positioned between the object and the light source such that the mask casts a shadow that covers the object and a portion of the area surrounding the object.
The method of the invention is well suited for depositing a layer of dielectric material over a device in which the dielectric layer has a via therethrough terminating on a metallic pad that is part of the device.